Meredith - It is just a liitle square but I thought the water key in the street is marked with W and a stop cock is just in your house (where the water comes in)... I am probably wrong though and willing to admit it 
 
TG - Ohhh I do hope it is your suggestion as we live in  terraced cottage and the septic tank is at the rear... the last owner had never had it emptied but said that they just put the pipe through the house 

  but my neighbours say that they have an access point.
Any further comments? or second opinions?